This invention relates to a heat-sensitive color transfer recording medium which is suitably used in thermal recording apparatus such as thermal transfer color printers, etc.
Recently, the thermal transfer color printers have passed beyond its purely academic and developmental stage to a practical stage and are coming into wide use rapidly. The thermal transfer color printers are used as color printers for computer aided design (CAD), computer aided manufacturing (CAM), and as color printers for full color copy machines.
Heat-sensitive color transfer recording mediums used in these thermal transfer color printers are composed of a substrate and a hot-melt ink layer.
As the substrate, a polyethylene terephthalate (PET) film of about 4 to about 7 .mu.m in thickness is suitably used. Usually, its surface to be brought into contact with a thermal head is provided with a heat-resistant layer to prevent sticking (fusing to the thermal head).
Hot-melt inks are usually applied onto the substrate sequentially in the order of yellow, magenta and cyan or in the order of yellow, magenta, cyan and black, and the heat-sensitive color transfer recording medium is used in roll form after being rolled round a cardboard core. Usually, the hot-melt inks are transferred in the order of yellow, magenta, cyan and black. The black hot-melt ink is used not only for transfer but also for a sensor.
The hot-melt inks are heated to the melting point or above by a very low applied energy from a thermal head to undergo phase change from solid to liquid, and transferred onto an image-receiving sheet to form images.
The hot-melt inks comprise coloring agents, solid waxes, resins, etc., and the blending proportions of these components greatly affect the transferability, the adherence to the substrate, the preservation at high temperatures (e.g. antiblocking) and the like, soiling of rollers in a printer, etc.
Whether the heat-sensitive color transfer recording medium can be used in a thermal transfer color printer or not is determined by a slight change of the blending proportions. For suitable employment of the heat-sensitive color transfer recording medium, its matching with a thermal transfer color printer is very important in practice.
For example, in U.S. Pat. No. 4,503,095, there is proposed a composition comprising 1 to 20% by weight of a coloring agent, 20 to 80% by weight of a binder (a solid wax) and 3 to 25% by weight of a softening agent (a resin or a lubricating oil). When a lubricating oil is incorporated into hot-melt ink, the ink is softened, so that rollers in a printer is solid and that printed images by transfer are liable to become dirty. Therefore, employment of no lubricating oil is preferred.
When a hot-melt ink containing 20 to 80% by weight of a solid wax is used as in the above-mentioned prior art reference, a yellow color as the first color tends to become blurred owing to badness in transfer at low temperatures (15.degree. C. or lower) in the case of starting printing by means of a thermal transfer color printer in which an applied energy to a thermal head is low. The second color and the subsequent color(s) are less liable to undergo badness in transfer than the yellow color by virtue of heat left in the thermal head. In the case of the color thermal transfer printer, printed letters of the three or four colors are superimposed, so that printing of a receiving sheet of about A4 size requires about 1 minute. Since high-speed printing is one sailing point of the color thermal transfer printer, the applied energy to the thermal head cannot be made very high, and when a hot-melt ink containing 20 to 80% by weight of a solid wax is used, printed letters of the first color, i.e., yellow tend to become blurred owing to badness in transfer.
Therefore, the color thermal transfer printer is designed to increase the applied energy to the thermal head at initial printing but has been disadvantageous in that sticking is caused at initial printing and that temperature control of the printer is complicated.